Field Test of Twin-Field Quantum Key Distribution through Sending-or-Not-Sending over 428 km

Quantum key distribution endows people with information-theoretical security in communications. Twin-field quantum key distribution (TF-QKD) has attracted considerable attention because of its outstanding key rates over long distances. Recently, several demonstrations of TF-QKD have been realized. Nevertheless, those experiments are implemented in the laboratory, and therefore a critical question remains about whether the TF-QKD is feasible in real-world circumstances. Here, by adopting the sending-or-not-sending twin-field QKD (SNS-TF-QKD) with the method of actively odd parity pairing (AOPP), we demonstrate a field-test QKD over 428 km of deployed commercial fiber and two users are physically separated by about 300 km in a straight line. To this end, we explicitly measure the relevant properties of the deployed fiber and develop a carefully designed system with high stability. The secure key rate we achieved breaks the absolute key rate limit of repeaterless QKD. The result provides a new distance record for the field test of both TF-QKD and all types of fiber-based QKD systems. Our work bridges the gap of QKD between laboratory demonstrations and practical applications and paves the way for an intercity QKD network with measurement-device-independent security.

Figure 1

(a) Bird’s-eye view of our field test. Alice is located at the Jinan Institute of Quantum Technology (JIQT) in Jinan (36°41’0.60” N, 117°8’10.93” E), while Bob is located at an internet data center (IDC) room in Qingdao (36°7’24.29” N, 120°27’11.88” E). The third-party measurement is done by Charlie in a room in Linyi (36°1’39.84” N, 118°44’50.58” E), which is 223 km from Alice and 205 km from Bob. Two yellow marks show the locations of two machine rooms at Yiyuan (36° 11’12.60” N, 118° 12’24.16” E) and Zhucheng (36°2’59.31” N, 119°24’43.58” E), respectively. An erbium-doped fiber amplifier (EDFA) is placed in each machine room to amplify the light for the clock and wavelength synchronization. Map data from Google, Landsat/Copernicus. (b) Illustration of the experimental setup. A continuous-wave (cw) bright beam from a 1550.12 nm master laser is multiplexed with the pulses from two 1570 nm auxiliary synchronization lasers (Sync Lasers) in Charlie and is transmitted along the synchronization channel. The slave laser of Alice and Bob is seeded by the cw bright beam and generates pulses with a width of 320 ps and a repetition rate of 312.5 MHz. The optical launch power of the slave laser is monitored in real time by a watchdog photoelectric detector PD2 of Alice (Bob). Then these pulses are sent to two sagnac rings SR1-2, which are randomly prepared in one of the four intensities strong ${\mu }_r$, high ${\mu }_{A2}$ (${\mu }_{B2}$), moderate ${\mu }_{A1}$ (${\mu }_{B1}$), and vacuum state. Three phase modulators PM1-3 are utilized for active phase randomization. The pulses are transmitted along the quantum channel and interfere in Charlie. The schematic of the polarization auto-alignment module is shown inside the red dashed rectangle. EVOA: electrical variable optical attenuator, FBG: fiber Bragg grating, CIR: circulator, EDFA: erbium-doped fiber amplifier, EPC: electric polarization controller, DWDM: dense wavelength division multiplexer, PBS: polarizing beam splitter.

Figure 2

Characterization of the crosstalk noise. All measurements are performed under the same overall detection efficiency (28%). (a) The crosstalk noise caused by the classical services running in some fibers in the optical cable. Without the master laser in the synchronization channel, we test two available fiber channels, fiber 1 and fiber 2 (fiber 3 and fiber 4), from Alice (Bob) to Charlie. The blue and red columns are the measurement results before and after filtering with two 100 GHz DWDMs, respectively. Note that we only need one fiber channel as the quantum channel to transmits the signal pulses from Alice (Bob) to Charlie. Taken the loss and the crosstalk noise of the fiber channel into account, we use fiber 1 (fiber 4) as the quantum channel from Alice (Bob) to Charlie. (b) The crosstalk noise caused by the cw bright beam in the synchronization channel with different optical launch power. Each experiment lasts 5 min. The experimental results are the average and variance (1 standard deviation) calculated by 144 experiments.

Figure 3

(a) Experimentally and simulated secure key rates. The secure key rate (purple pentagram point) of our work is $4.80\times {10}^{-8}$/pulse, corresponding to 3.36 bps. The red curve is the simulation results using the parameters in Table I of Ref. [37]. The green and blue dashed line are the absolute PLOB bound (assuming the overall detection efficiency of Charlie ${\eta }_d=1$) and the relative PLOB bound with ${\eta }_d=28\%$, respectively. (b) The bit error rate in $X$ windows. Each data point represents the effective clicks collected in 21.82 min on average. (c) The probability distribution of the reflectivity for the PBSs in Charlie, with real-time compensation in the overall experiment. The total efficiency of the polarization auto-alignment module is about 94% (the detail see Ref. [37]). (d) The probability distribution of the overlapping rate between the signal pulse and detection window in the overall experiment. The total overlapping is 70% (see details in Ref. [37]).